(Part 1) 1.21 Antimicrobial susceptibility Testing Flashcards
The goal of AST:
Determine whether the bacterial etiology of concern is capable of
expressing SUSCEPTIBILITY to the antimicrobial agents that are potential choices of therapy
T or F
F
RESISTANCE
Lowest concentration of antimicrobial that completely inhibits visible growth
Minimum inhibitory concentration (MIC)
Antimicrobial agent in question has ossible effectivenes against the isolate, but possible less
Intermediate
Antimicrobial agent in question may be an appropriate choice for treating the infection
Susceptible
Antimicrobial agent in question may not be an appropriate choice for treatment
Resistant
Altering dosing results in higher drug exposure
Susceptible dose dependent SDD
The antimicrobial agent with MICs above a susceptible breakpoint
Nonsusceptible
Standard medium for any type of susceptibility test
Mueller Hinton agar
What are the 2 important req for Inoculum prep?
Use of pure culture
Use of a standard sized inoculum
Incubation for inoculum prep?
3-5 hrs at 35-37C
Compare the turbidity of bacterial suspension with ?
0.5 McFarland standard
Components of 0.5 McFarland standard
1% Sulfuric acid
1.175% barium chloride
Matching the turbidity with 0.5 McFarland standard provides an optical density comparable to the density of a bacterial suspension of ______________
1.5x10^8 CFU/mL
Methods used in AST:
Testing involves challenging the organisms of interest with antimicrobial agents in a liquid environment
Borth dilution: tube or microplate
Methods used in AST:
Antimicrobials and organisms to be tested are brought together on an agar-based medium
Agar dilution
In broth dilution, each antimicrobial agent is tested using a range of concentrations expressed as _______ of broth
mg/mL
Methods used in AST:
Uses antibiotic diks placed on the surface of an agar plate seeded with a lawn of the bacterial isolate
Disk Diffusion (Kirby Bauer)
The Zone of inhibition is directly proportional to the MIC
T or F
F
Inversely proportional
How many mm of agar should be used in kirby bauer?
150 mm
How many disks should be used in kirby bauer?
12
How thick should the agar be in Kirby bauer?
4mm
Incubation for kirby bauer
35C for 18-24 hrs
As Ca2+ and Mg2+ increases ____________ becomes more resistant to aminoglycosides
P. aeruginosa
What is the result for sulfonamides (Susceptible)
2 concentric rings around the disk
Inoculum is too light
Very dry agar surface
<4mm agar depth
Higher disk concentration
> 15 mins delay after disk is applied and before incubation
False sensitive
Inoculum is too heavy
Too much moisture on the agar
Agar depth >4mm
Deteriorating antibiotic disks
> 15 mins delay after seeding the plate and before antibiotic disks are applied
False resistant
In E-Test, “E” stands for?
Elliptical
This test uses strips with varying concentrations of antibiotic along its length
E test
How is MIC is taken in E test?
Growth inhibition edge intersects
Principle of VITEK system?
Turbidimetry
What is the substrate used in Beta lactamase Production
Cephalosporin
Involves continuation of conventional broth dilution testing
MBC
Refers to the concentration resulting to a 99.9% reduction in CFU per milliliter
MBC
Involves exposing a bacterial isolate to a concentration of antibiotic n a broth medium and measuring the rate of killing over a specified time period.
Time-kill studies
Analogous to the MIC-MBC test except that the test medium used is the patient’s serum containing the therapeutic antimicrobial agents the patient has been receiving
Serum bactericidal test (Schlicter Test)
In Schlicter test:
This specimen is collected after the antimicrobial agent is given
Peak specimen
In Schlicter test
This specimen is collected just before the patient is to receive the next antimicrobial dose
Trough specimen
The highed dilution the inhibits visibly detectable growth is the
serum-static titer
Serum dilution resulting in a 99.9% reduction in the CFU/mL is recorded as
Serum-cidal titer
